Early childhood infection and allergies lead to substantial morbidity and health costs. Accumulating evidence suggests an important role of vitamin D in immunity. However, the studies relating vitamin D status to risk of atopy/allergy and infection in childhood are inconsistent; there are reports of both increased and decreased health risks according to vitamin D status. Moreover, there are few studies that explore the potential interaction between Vitamin D Receptor {VDR) genotypes, vitamin D status, and childhood immune outcomes. We aim to study how maternal and cord blood vitamin D concentration and infant VDR genetic profile are associated with immunity in the first three years of life within a large prospective birth cohort study. One thousand mother-child pairs are currently being enrolled in the New Hampshire Birth Cohort Study (NHBCS), a study that collects extensive data on nutritional, medical, environmental, and sociodemographic factors, as well as several biological specimens before and after delivery. The NHBCS will serve as a resource for biospecimen and data collection through questionnaires, follow-up interviews and medical record reviews. As part of the proposed study, we will measure maternal and cord blood concentrations of 25-hydroxyvitamin D (25(OH)D) and infant VDR genotypes in the NHBCS. First, we will study the associations between 25(OH)D concentrations with immune outcomes during the first 3 years of life. Atopy/allergy outcomes will include all diagnoses of atopy/allergy and specifically: atopic dermatitis, food and environmental allergies, allergic rhinitis, wheeze and asthma. Infection outcomes will include all infection diagnoses, and specifically respiratory and gastrointestinal infections. Analyses will evaluate potential confounding by factors such as gestational age at delivery, adiposity, and breast-feeding/formula feeding. Next, we will explore whether VDR polymorphisms are independently associated with the atopy/allergy and infection outcomes and whether they modify the association between maternal and cord blood 25(OH)D concentration and those outcomes. The goal of this work is to clarify the association between vitamin D status, a factor that is highly modifiable through supplementation of mothers and/or infants, with child health outcomes in the context of relevant environmental and genetic factors.